The present invention relates, in general, to measuring time, and more particularly, to measuring a time interval between two events using a time to digital converter.
In some applications such as electronic circuit testing, radar ranging, and elementary particle physics, it is often indispensable to be able to measure time at a very high resolution. To achieve a resolution finer than the period of a clock signal, a common approach uses an analog ramp circuit. The analog ramp circuit generates voltage ramp signals, which are used in conjunction with the clock signal to measure a time interval between a start signal and a stop signal. More particularly, the start signal activates a first ramp circuit, which in turn generates a first voltage ramp signal. The voltage increases until the first ramp circuit is deactivated by a leading edge of the clock signal following the start signal. At some time after the start signal, the stop signal activates a second ramp circuit, which in turn generates a second voltage ramp signal. The voltage increases until the second ramp circuit is deactivated by a leading edge of the clock signal following the stop signal. The slope of the each voltage ramps is equal to the rate of the voltage increase with respect to time. The duration of the first voltage ramp and the duration of the second voltage ramp are calculated by dividing the voltage excursions of the first and second voltage ramps by their respective slopes. In addition, the time duration between the leading edge of the clock signal following the start signal and the leading edge of the clock signal following the stop signal is calculated using a counter. This duration is referred to as a clocking interval. The time interval between the start signal and the stop signal is calculated by subtracting the duration of the second voltage ramp from the sum of the duration of the first voltage ramp and the clocking interval.
Circuits used for generating and sensing linear voltage ramps are typically large and complex, and are, therefore, expensive to manufacture on a monolithic integrated circuit. Furthermore, the circuits are not sufficiently accurate for some high resolution measurements. In addition, the cost of building measuring devices using the analog ramp circuits is usually high.
Accordingly, it would be advantageous to have a simple and inexpensive circuit for measuring time and achieving a resolution finer than the period of a clock signal. It is also desirable for the circuit to convert the measured time to a digital value quickly and accurately. It would be of further advantage for the circuit to be sufficiently small to be manufactured as a monolithic integrated circuit.